Resonating circuits are important in microwave radar and communication circuits for generating, filtering and selecting frequencies in oscillators, amplifiers and tuners. Some microwave oscillators employ a small slab of dielectric material, referred to as a dielectric resonator, adjacent to an electrical conductor in order to create resonance at a selected frequency of current carried by the conductor. The resonant frequency of the dielectric resonator is dependent upon the geometry of the conductors in its vicinity. The resonant frequency of the dielectric resonator may be manually adjusted by altering the resonator's proximity or orientation with respect to the chamber within which it is located. Therefore, the apparatus for mounting a dielectric resonator within a resonator cavity is of primary importance.
Typically, the resonator is glued in position within the resonator cavity. For example, U.S. Pat. No. 4,963,841 to Sparagna discloses a "Dielectric Resonator Filter" in which a puck-shaped dielectric resonator is bonded directly to the base of the resonator cavity by a thin film of adhesive. The resonator is positioned by a plastic screw which engages a centrally located, threaded aperture in the resonator. The screw further engages a similar aperture in the base of the resonator cavity.
There are several problems with this prior art method. First, the epoxy or glue must cure before the part can be moved down the manufacturing line, thereby increasing manufacturing cycle time. Second, the epoxy thickness will vary, affecting the resonator frequency. Third, the dielectric constants of the resonators vary from lot to lot, requiring precise adjustments in resonator height. Such precise adjustments are difficult to accomplish with adhesives involved. Last, the application of adhesives is not easily accomplished in an automated procedure.
Mechanical means to constrain and position a resonator have also been developed. For example, U.S. Pat. No. 5,034,711 to Hendrick et al. discloses a "Dielectric Resonator Support System for a Waveguide". A dielectric resonator is held in position using support posts or rods. The support rods are made from a suitable dielectric material, and slip-fitted in holes provided at ninety degree intervals around the periphery of the dielectric resonator. The support rods are affixed to the waveguide, by gluing or being screwed in place. The support rods are slip-fitted such that they are allowed to expand, due to temperature, without being affixed to its supporting structure. However, placement of these rods and their connection to the resonator are delicate operations which are unsuitable for an automated assembly system.
Another example of mechanical resonator constraint means is disclosed in U.S. Pat. No. 4,939,489 to Gueble et al. entitled "Filter Having A Dielectric Resonator." This filter comprises at least one cylindrical cavity containing a cylindrical dielectric resonator whose axis of symmetry is collinear with the axis of the cavity. The resonator is held in a longitudinally asymmetrical position inside the cavity by a mandrel system which clamps around the cylindrical portion thereof and which includes at least one spoke for fixing it to said cavity while leaving play relative to said cavity. Again, however, this mandrel apparatus requires extensive manual installation and adjustment and is unsuitable for use in an automated process.
A need exists for an apparatus and method for mounting a resonator in a resonator cavity. The method should not require the use of epoxies or glues to constrain the resonator in position. The apparatus should allow for precise, positive placement of the resonator within the resonator cavity and yet be suitable for use in an automated assembly process. Such a resonator mounting system must also produce consistently desired resonator behavior.